Interpretive Summary: Postmortem storage at refrigerated temperatures has been clearly demonstrated to improve meat tenderness. Freezing and then thawing meat has been shown to either improve tenderness, increase toughness or have no effect at all on meat tenderness. The use of hydrodynamic pressure processing in conjunction with freezing meat either before or after HDP treatment was evaluated. Use of HDP, whether on fresh or frozen/thawed meat, successfully tenderized meat. Whether one freezes the meat before performing HDP or after HDP treatment, both successfully improve meat tenderness and the improvements are maintained even after frozen storage. Although freezing alone did show sign of improving meat tenderness, the magnitude of improvement was much less than the tenderness improvements found when using HDP technology.

Technical Abstract:
Two experiments were conducted to determine the effect of freezing time postmortem on the effectiveness of the hydrodynamic pressure processing (HDP) technology to tenderize beef. In Exp. 1, the effect of hydrodynamic pressure processing (HDP) using two different shaped explosive charges (rectangular [REC] vs cylindrical [CYL]) and meat state at 48 h postmortem of beef strip loins (N=16) on meat tenderness was evaluated. HDP was performed by detonating 100 g of explosive placed above vacuum packaged meat samples immersed in water in plastic containers. Meat state was fresh, never frozen compared to frozen at 48 h postmortem, followed by thawing at 6-d postmortem. Meat samples were evaluated for tenderness by shear force measurements at both d-1 and d-6 after being treated with HDP. Both shapes of explosives improved (P<0.01) shear force on d-1 compared to controls. The effect of HDP was sustained (P<0.01) throughout d-6 of aging, with the CYL reaching 3.9 kg and the REC reaching 3.6 kg compared to controls (4.5 kg). Freeze/thaw samples were 1 kg lower at d-1 compared to fresh samples and 0.8 kg lower at d-6. In Exp. 2, two 2.5 cm thick control steaks were removed from the loin end of forty boneless, ribeye sections at 5 days postmortem. One control steak was cooked fresh while the other was frozen for 30 days and later cooked after thawing for shear force determination. Ten cm thick pieces from the ribeye sections were packaged for HDP processing. HDP (100 g binary explosive rectangular shape) was performed on the 40 fresh ribeye sections. Twenty of the HDP treated ribeyes were frozen after HDP treatment. Two 2.5 cm thick steaks were removed from the remaining fresh, 20 HDP treated samples 24 h after treatment and cooked for shear force determination. Sixty days after HDP treatment the frozen samples (both control steak pieces and HDP treated pieces) were thawed, cooked and evaluated for shear force. Control (never frozen) steaks prior to HDP treatment had shear values of 6.5 kg. A 27 percent reduction in shear force was observed for HDP treated (fresh) samples. Freezing accounted for a 14 percent reduction in shear force. HDP treatment followed by freezing resulted in a 29 percent reduction in shear force compared to the frozen control samples. These results suggest that both HDP and early postmortem freezing followed by thawing are successful treatments for improving meat tenderness and are better than extended conventional aging. Furthermore, HDP treatment yields instantaneous improvements in tenderness and the improvements are maintained even after freezing.